Aluminum alloys containing beryllium and investment casting of such alloys

ABSTRACT

Disclosed is a practical aluminum-based alloy containing 1 to 99 weight percent beryllium and improved methods for the investment casting of net shape aluminum-beryllium alloy parts.

This is a divisional of application Ser. No. 08/221,395 filed Mar. 31,1994, which is a continuation-in-part of application Ser. No. 08/156,356filed Nov. 23, 1993 and now abandoned, which is a continuation ofapplication Ser. No. 07/770,187 filed Oct. 2, 1991 and now abandoned.

FIELD OF INVENTION

The present invention relates to alloys of beryllium and aluminum. Moreparticularly, the invention describes a method for making alloys ofaluminum containing beryllium and forming them into useful structuralproducts by investment casting techniques.

BRIEF DESCRIPTION OF THE PRIOR ART

Alloys of aluminum and beryllium are known in the art. For example,Cooper U.S. Pat. No. 1,254,987 describes the addition of aluminum toberyllium for improving machinability. Fenn U.S. Pat. No. 3,337,334discloses and claims the Lockalloy commercial product (developed byLockheed and Berylco in the 1960's) which comprises aluminum base metaland 62 weight percent beryllium.

Lockalloy was produced in sheet form and incorporated into the ventralfin of the YF12 experimental aircraft (Duba, YF-12 Lockalloy Ventral FinProgram, Final Report, NASA CR-144971, 1976). Following the introductionof Lockalloy, extensive data was obtained on rolled alloys made frompre-alloyed aluminum having 62 weight percent beryllium. See, forexample, London, Alloys and Composites, Beryllium Science andTechnology, Volume 2, Plenum Press, New York (1979).

Second and third order elemental additions to aluminum-beryllium alloysare reported in the literature. They include additions of magnesium,silicon, nickel or silver for making ternary and quaternary alloys ofaluminum and beryllium as described in McCarthy U.S. Pat. No. 3,664,889.These alloys are made from rapidly solidified alloy powder, consolidatedand worked by conventional means. Russian work on ternary and higherorder aluminum-beryllium alloys is variously described in Molchanova,Phase Equilibria in the Al--Be--Ni System at 600 Deg. C, Vest. Mosk.Univ. Khim., Vol. 27(3), pages 266-271 (1986); Komarov, Increasing theStrength of Welded Joints in an Al--Be--Mg Alloy by Heat Treatment,Weld. Prod., Vol. 26(1), pages 32-33 (1979); Kolachev, ConstructionalAlloys of Aluminum Beryllium and Magnesium, Metalloved. Term. Obrab.Metal. Vol. 13, pages 196-249 (1980); Nagorskaya, Crystallization inAl--Be--Mg--Zn Quaternary System Alloys, Metalloved. Term. Obrab.Metal., Vol. 9, pages 72-74 (1973).

Minor amounts of beryllium are typically added to aluminum-rich alloysto prevent oxidation of the aluminum and other alloy components duringprocessing steps like melting and pouring. As a primary example, BrushWellman Inc., Elmore, Ohio produces and distributes aluminum-rich masteralloys containing 10 percent or less beryllium for further processing bybulk producers. The residual beryllium level in downstream aluminumproduct is preferably less than 0.01 percent.

The most current aluminum-beryllium phase diagram shows a simpleeutectic with essentially no terminal, solid solubility at either end.This Al--Be phase diagram, adopted from Murray, The Aluminum-BerylliumSystem, Phase Diagrams of Binary Beryllium Alloys, ASM InternationalMonographs on Alloy Phase Diagrams, page 9 (1987), is reproduced as FIG.1 in this specification.

Brush Wellman has conducted extensive research on aluminum alloyscontaining from about 10 to about 75 weight percent beryllium. SeeHashiguchi, Aluminum Beryllium Alloys for Aerospace Application,European Space Agency Structural Materials Conference, Amsterdam (March1992). The research showed that an aluminum alloy of about 62 weightpercent beryllium is about 70 volume percent beryllium, and an alloy of50 weight percent beryllium is about 59 volume percent beryllium. It wasalso discovered that the density and elastic modulus of alloycompositions in this system follow the Rule of Mixtures, i.e.,interpolation of alloy properties is generally possible between therespective properties of pure beryllium and pure aluminum.

Results from studies at Brush Wellman's Elmore facilities have alsoshown that large cast ingots and fine pre-alloyed atomized powderparticles can be produced with microstructures showing a metal compositeincluding beryllium in an aluminum matrix. Presently, Brush Wellmanmarkets these alloys as extrusions and stamped sheet products under thetrademark AlBeMet™.

Brush Wellman has processed AlBeMet™ into useful component parts by twoalternative routes. Both processes require vacuum melting of aluminumand beryllium starting materials in a ceramic-lined, refractory crucibleat temperatures typically in the range between about 1350° to about1450° C. In the first alternative, the liquified aluminum-beryllium meltis poured through a refractory nozzle to produce a stream which isintercepted by high velocity jets of an inert gas. The jets of gas breakthe liquid stream into tiny grains which solidify into a pre-alloypwder. Individual grains that comprise the powder pre-alloy have veryfine dendritic microstructure consisting of a beryllium phase within analuminum alloy matrix. The pre-alloy powder is then consolidated by coldisostatic pressing, hot isostatic pressing or extrusion to produce agross shape which can then be machined into a useful article.

The second alternative for processing AlBeMet™ into component parts is aconventional ingot casting operation in which molten aluminum-berylliumis poured into a graphite mold cavity and cooled to a solid ingot up tosix inches in diameter. The microstructure of this casting is arelatively coarse, dendritic beryllium phase within an aluminum alloymatrix. The casting surface and hot-top are removed and scrapped and theingot is further processed by rolling, extrusion or machining into thefinal article shape. These alternatives are relatively expensive andcheaper net shaping processes are preferred.

Investment casting is a sub-set of precision metal processing whichproduces net shape parts to reduce subsequent machine losses. Adisposable ceramic shell patterned after the intended. structure is usedas a mold for casting metal alloy articles. See Horton, InvestmentCasting, Metals Handbook, 9th Ed., Vol. 15, pages 253-287 (1984). Moltenalloy is poured into the mold, an article is fabricated and the ceramicshell is destroyed as it is separated from the cooled metal alloy part.

Prior to the present disclosure, there have been no reports ofinvestment casting for aluminum-beryllium alloys since conventionalknowledge predicts great difficulty for investment casting any alloywith a large differential between liquidus and solidus temperatures asfound in the aluminum-beryllium alloy system (see FIG. 1). The largedifference between the liquidus an solidus temperatures of aluminumalloys containing the most useful beryllium levels theoretically makescasting these alloys very difficult or nearly impossible. For instance,an art-recognized casting defect known as "hot tearing" increases withdifferences between the liquidus and solidus temperatures of the castalloys. See Davies, Contraction Cracks, Solidification and Casting,pages 174-176, Applied Science Publishers, Essex, England (1973).

The present specification describes solutions to the stated problems formaking alloys of aluminum containing beryllium and further discloses animprovement for investment casting of metal. alloys.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide practical net shapeparts of aluminum-based alloys with beryllium additions in the range of1 to 99 weight percent by modified investment cast processing.

It is also an object of the present invention to provide practical netshape parts of an aluminum-based alloy with beryllium additions,preferably in the range of 55 to 80 weight percent.

It is another object to provide a method for investment casting whichselectively employs alloying elements for improving the castability andproperties of the resulting net shape parts.

A further object of the present invention is to provide an improved,cost-effective investment casting method for producing complex shapes ofan aluminum-based alloy with beryllium additions, preferably in therange of 55 to 80 weight percent.

Another object is to provide a production method which uses near netshape dies to reduce machining costs.

It is yet another object to provide a method by which precision, netshape aluminum components can be formed with significant amounts ofberyllium.

Other objects of the present invention will become apparent to thoseskilled in the art after a review of the following disclosure.

SUMMARY OF THE INVENTION

The current state of the art for fabricating structures fromaluminum-beryllium based alloys is directed to powder metallurgy.Pre-alloyed powder is atomized, consolidated and subject to standardmetal working practices to produce a blank for machining into the finalpart.

The present disclosure teaches precision investment casting ofaluminum-based alloys containing significant amounts of beryllium toproduce practical net shape aluminum-beryllium components directly fromraw input materials. The term "net shape" as used in this specificationdescribes a component which is very near its final form, i.e., aprecision casting that requires very little further machining beforeend-use application.

This invention successfully uses investment casting to manufacturealuminum alloys containing beryllium. The presenting claimed alloys (andcorresponding parts) have densities lower than other known aluminumalloys and a modulus of elasticity nearing that of beryllium. Themodulus increases with beryllium content and approaches a linearcombination between the modulus of aluminum at 10.0 million psi and themodulus of beryllium at 44 million psi.

The following table summarizes the properties of the variousberyllium-containing aluminum alloys made according to the presentinvention.

                  TABLE I                                                         ______________________________________                                        Beryllium-Containing Aluminum Alloy Property Comparison                       Be     Density  Modulus  E/Rho   CTE                                          (Wt %) (lb/in.sup.3)                                                                          (MSI)    (in × 10.sup.6)                                                                 (in/in/°F. × 10.sup.-6)         ______________________________________                                        0      0.097    10.0     102.6   13.1                                         5      0.095    12.4     130.5   12.6                                         10     0.093    14.7     158.3   12.2                                         15     0.091    17.0     186.2   11.7                                         20     0.089    19.1     214.0   11.3                                         25     0.087    21.1     241.9   10.9                                         30     0.086    23.1     269.7   10.5                                         35     0.084    25.0     297.6   10.2                                         40     0.082    26.8     325.4   9.8                                          45     0.081    28.5     353.3   9.5                                          50     0.079    30.2     381.1   9.1                                          62     0.076    33.9     448.0   8.4                                          70     0.074    36.3     492.5   7.9                                          80     0.071    39.0     548.2   7.4                                          90     0.069    41.6     603.9   6.9                                          100    0.067    44.0     659.7   6.4                                          ______________________________________                                    

The commercial market requires aluminum based alloys with higher elasticmodulus and lower density. As indicated in Table I, a continuousvariation of properties from those of the aluminum alloy at one extremeto beryllium at the other is achieved. For example, a 5 percentberyllium increment produces a 25 percent higher modulus at about thesame density when compared to the aluminum alloy base.

Investment casting of aluminum and beryllium offers previously unknownlatitude for selecting the size and shape of component parts. Accordingto the present invention, highly porous net shape parts require verylittle machining to arrive at the final product. As a result, labor andmaterial costs are dramatically reduced when compared to products whichare "hogged out" from a bulk shape.

The present invention has universal application to a wide variety ofparts including, but not limited to, aerospace fuselages, emergency doorlatches, steering columns, engine pylons, support structures, wingstabilizers, rotor swashplates, avionic boxes, turbine engines,manifolds, gear boxes, diffusers, particle separators, oil tanks,stators, compressors, pumps, hydraulic equipment, electronic packaging,electrooptical components, computer and disk drive hardware, sportingequipment and the like.

A full description of the present invention will now be provided withreference to the figures and examples that follow.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a current aluminum-beryllium phase diagram.

FIG. 2 is an x-ray radiograph of an investment cast aluminum-berylliumdisk drive arm made according to the present invention.

FIG. 3 is an avionics box investment cast from an aluminum-berylliumalloy made according to the present invention.

FIG. 4 illustrates an assembly of read/write heads comprised of thepresently disclosed aluminum-beryllium alloy.

FIG. 5 shows a net shape, single actuator arm from the assembly of FIG.4. Forces exerted on the arm are represented by vectors.

DETAILED DESCRIPTION OF THE INVENTION

The examples below were conducted to produce net shapes of aluminumalloys containing additions of beryllium. Such aluminum-beryllium alloyswere produced into net shapes through investment casting following theselected parameters. The examples clearly demonstrate that investmentcasting of an aluminum alloy with significant amounts of beryllium issuccessful according to the present methods.

All environmental health and safety equipment, including supplementaryHEPAVAC ventilation, are installed prior to the initiation of trials.Air counts are taken periodically during the trials and final clean-upoperation. All participants wear suitable air filter masks and clothingduring the trials. Further safety details are available from BrushWellman Inc., Cleveland, Ohio.

EXAMPLE 1

Investment Casting of an Aluminum-Beryllium Alloy Part

An alloy charge weighing 8 pounds with a composition of 38 weightpercent clean aluminum rod and 62 weight percent clean beryllium lumpwas placed in an induction-heated alumina-magnesia crucible. Aluminumrod, with 99+ percent purity, was obtained from Alfa Johnson Mathey,Ward Hill, Massachusetts and Grade B-26-D beryllium was supplied byBrush Wellman. The crucible was situated inside a water-cooled steelchamber which could be evacuated to a vacuum of 1×10⁻⁴ torr. Alsolocated in the steel chamber was a resistance-heated preheat furnacecontaining a ceramic shell mold. The mold was fabricated by dipping aberyllia (BeO) slurry over a wax pattern which consisted of three rodsattached to each other by a sprue. A commercial part is manufactured bysubstituting a wax pattern which matches the configuration of interest.

The vacuum furnace was equipped with an optical pyrometer to measure thetemperature of the melt and a thermocouple to measure the temperature ofthe ceramic mold. Power to the preheat furnace was turned on at 600° C..When the temperature of the mold reached 600° C. (total time about 16hours), the induction field was activated and the aluminum charge meltedunder vacuum of approximately 0.1 torr. Total time between power-on andmelting was two hours. Once molten, the temperature of the liquid metalwas increased to 1375° C. to provide superheat to the melt. During thisperiod of time, the vacuum increased to 0.8 torr because of outgassingfrom the melt. The melt was maintained at 1375° C. for five minutes toprovide uniform heating and stirring of the melt. After the hold period,the melt was poured into the ceramic mold where it solidified.

After casting, all power to the crucible and mold preheat furnace wasshut off and the mold cooled to room temperature overnight. Once cool,the ceramic was separated from the now solidified aluminum-berylliumalloy using a hammer and sandblasting unit. The bars were cut from thesprue and samples were prepared for metallographic and mechanicalproperty analysis. The microstructure of the cast alloy consisted ofberyllium dendrites surrounded by an aluminum matrix. Several smallregions of porosity were also observed in the microstructure. Tensilesamples machined from the other bars were not tested because of porosityin the microstructure.

EXAMPLE 2

Investment Casting of an Aluminum-Beryllium Disk Drive Arm

To demonstrate the principles of the present invention, a net shape diskdrive arm was investment cast from aluminum and beryllium. The resultingdrive arm is shown in the x-ray radiograph presented as FIG. 2.

A wax pattern was specified by Brush Wellman, and designed by PrecisionCastparts Corporation, Minerva, Ohio, to simulate a four-fingered drivearm. This four-fingered configuration was selected to demonstrate theversatility of the present invention. Two wax patterns were joined sothat two parts could be recovered from a single pour. The wax was coatedto make a ceramic casting mold and removed using the "lost-waxtechnique" which is well known in the art.

The mold was placed in a vacuum casting furnace and electricallypreheated. An aluminum alloy containing 62 weight percent beryllium wasmelted in the vacuum furnace and poured into the mold as described inExample 1. After cooling, the ceramic mold was chipped off the casting,leaving two well formed drive arms and associated gating. The cast partswere x-rayed and superior integrity was confirmed by the radiograph ofFIG. 2.

EXAMPLE 3

Investment Casting of Parts

The procedures outlined in Examples 1 and 2 were followed to make theavionics box illustrated in FIG. 3. This box has all the characteristicssuitable for modern aircraft, including high stiffness, good mechanicalsupport, low weight and excellent heat removal characteristics, with acoefficient of thermal expansion low enough to ensure stability duringtemperature cycling.

The methods of Examples 1 and 2 were also followed to make thestructures shown in FIGS. 4 and 5. These figures illustrate a rotatablearmset of an actuator having a bore for rotating about the shaft of adisk drive for positioning a head radially across a disk, wherein thearmset is a one piece unit consisting essentially of an alloy ofaluminum containing from about 1 to about 99 weight percent berylliummade by investment casting.

In particular, FIG. 4 illustrates a read/write assembly for hard diskdrive 10 having multiple heads 12 mounted on actuator arms 14. Heads 12and actuator arms 14 are assembled together on actuator shaft 16 whichis rotated by the interaction of wire coil 18 and magnet 20 disposed inmagnet housing 22. Actuator arms 14 are spring loaded to rest on thedisk when it is stationary. When the disk is rotated, air pressuredevelops beneath head 12 and lifts it slightly above the disk.

Actuator arms 14 are subjected to vertical forces 24 and angular forces26 as shown in FIG. 5. Actuator arms 14 should be sufficiently stiff tominimize the amplitude of vertical vibration and avoid damaging thedisks above and below actuator arms 14. Likewise, actuator arms 14should be sufficiently stiff to minimize the amplitude of lateralvibration and provide a more rapid response time for reading or writingat an appropriate address on the disk. Laminated materials are effectivein minimizing deflections principally in the vertical direction. Thealuminum-beryllium alloy made according to the present invention iseffective to minimize deflections in both the vertical and lateraldirections.

EXAMPLE 4

Investment Casting a Ternary Al--Be--Ni Alloy

An alloy charge weighing 10 pound was produced with a composition of 35weight percent clean aluminum rod, 62 weight percent clean berylliumlump and 3 weight percent nickel pellets (99.7 percent pure, obtainedfrom Alfa-Johnson Mathey). The charge was placed in an induction-heatedalumina-magnesia crucible located in the vacuum furnace described inExample 2. A shell mold placed in the resistance-heated preheat furnacewas patterned after sixteen tensile test bars. For commercialapplication's, the test bars are replaced with end-use configurationssuch as the avionics box described above.

Using the resistance-heated preheat furnace, the mold temperature wasincreased to 700° C. over a period of about 16 hours. The inductionfield was activated and the aluminum, beryllium and nickel charge wasmelted under vacuum of approximately 0.1 torr. Total time betweenpower-on and melting was two hours. Once molten, the temperature of theliquid metal was increased to 1375° C. to provide superheat to the melt.The melt was maintained at 1375° C. for five minutes to provide uniformheating and stirring of the melt. During this period, argon gas was bledinto the furnace chamber until the pressure reached one atmosphere. Thealloy melt was then poured into the ceramic mold.

After pouring, the power to the crucible and preheat furnace was shutoff and the metal-filled ceramic mold was allowed to cool overnight.Once cool, the ceramic was separated from the aluminum-beryllium-nickelalloy casting using a hammer and sandblasting unit. The tensile barswere cut off using a band saw, and samples were cut from the gating formetallographic analysis.

The microstructure of the cast alloy consisted of beryllium. dendritessurrounded by an aluminum matrix. Examination of the specimen in ascanning electron microscope, equipped with an energy dispersive x-raycapability, indicated that the nickel alloying addition had migrated tothe beryllium phase. Porosity was still observed in the microstructure,but the volume fraction of porosity was decreased. Tensile propertieswere measured for several test bars. The 0.2 percent yield strength wasfound t be 22,000 psi, the ultimate tensile strength was 25,000 psi andthe elongation was 2.1 percent.

The cast parts fabricated in this example were placed in a hot isostaticpress (HIP) and heated to 450° C. for two hours, while a pressure of15,000 psi was applied. Metallographic analysis of the parts after thistreatment revealed that the combination of temperature, time andpressure eliminated most of the porosity not connected to the surface.

EXAMPLE 5

Investment Casting of Higher Order Aluminum-Beryllium Alloys

Alloys of aluminum and beryllium containing other elements can befabricated using the process outlined in Example 3. The alloycomposition may be represented by the following formula: (30-75%Be)+(25-70% Al)+(0.25-5% X)+(0-5% Y)+(0-0.5% Z) where the letters X, Yand Z designate elements listed below in Table II and the total weightof alloy components must equal 100 percent.

                  TABLE II                                                        ______________________________________                                        Alloying additions for Aluminum-Beryllium alloys                              ______________________________________                                        X -     Nickel, Cobalt, Copper                                                Y -     Silver, Silicon, Iron                                                 Z -     Titanium, Zirconium, Boron, Scandium, Yttrium,                                and all elements considered rare earth elements.                      ______________________________________                                    

For example, the components for a 10 pound alloy charge with acomposition of 30 weight percent aluminum rod, 64 weight percentberyllium lump, 3 weight percent nickel, 1.5 weight per-cent silver and1.4 weight percent silicon were placed in an induction-heatedalumina-magnesia crucible located in the vacuum furnace described inExample 2. An addition of 0.1 weight percent titanium is placed in ahopper for addition to the melt just prior to casting. A shell mold toreceive the molten alloy is placed in a resistance-heated preheatfurnace. The mold may match the configuration for tensile bars,engineering shapes, sports equipment and the like.

Using the preheat furnace, the mold temperature is increased to between350° C. and 1275° C. The exact temperature depends on the mold shape andalloys cast. If a mold preheat unit is available, the mold may be heatedoutside the furnace and placed into a casting chamber just prior topouring. This casting chamber should be separated from the meltingchamber by a vacuum tight valve which may or may not have its own heatsource.

When the mold reaches the selected preheat temperature, the inductionfield is activated and the components of the aluminum-beryllium alloyare melted together. The vacuum during melting must not be lower than0.0001 torr or excessive vaporization of the alloying elements willoccur. Once molten, the temperature of the liquid metal was increased tono more than 1500° C. to provide superheat to the melt. One minute priorto pouring, titanium is added to the melt to promote fine grain andproduce a dispersion of fine, hard intermetallic particles in the finalproduct. One minute after the titanium is added, the melt is poured intothe ceramic shell mold.

After pouring, the power to the crucible and preheat furnace was shutoff and the metal-filled ceramic mold was allowed to cool to roomtemperature. Alternatively, the hot mold can be removed from the furnacefor cooling. Once cool, the ceramic was removed from thealuminum-beryllium alloy casting using mechanical or chemical methods,or a combination thereof. The useful parts are removed from the castingfor further processing.

Higher order alloys like those described in this example can be improvedfor strength and ductility by one or more heat treat processes which arewell known in the aluminum alloy art. A hot isostatic pressing (HIP)step, as described in Example 3, can be used either before or after theheat treatments.

Various modifications and alterations to the present invention may beappreciated based on a review of this disclosure. These changes andadditions are intended to be within the scope and spirit of thisinvention as defined by the following claims..

What is claimed is:
 1. A method for making an aluminum alloy containingberyllium comprising the steps of:(a) providing a solid aluminumcomponent and a solid beryllium component to form an alloy charge; (b)melting the charge of step (a) in a refractory-lined furnace pot withina vacuum melting furnace; (c) pouring the liquid melt from step (b) intoa disposable shell mold; (d) freezing said melt within said disposableshell mold; and (e) removing said disposable shell mold.
 2. The methodof claim 1, wherein the resulting aluminum alloy containing berylliumhas a modulus of elasticity at least 25 percent higher than that ofaluminum.
 3. The method of claim 1, wherein the resulting aluminum alloycontaining beryllium comprises about 5 to about 80 weight percentberyllium.
 4. The method of claim 3, wherein said resulting aluminumalloy containing beryllium comprises about 5 to about 80 weight percentberyllium dispersed in substantially pure aluminum.
 5. The method ofclaim 3, wherein said solid aluminum component of step (a) is analuminum-rich composition and said resulting aluminum alloy containingberyllium comprises about 5 to about 80 weight percent berylliumdispersed in said aluminum-rich composition.
 6. The method of claim 5,wherein said aluminum-rich composition contains an element selected fromthe group consisting of magnesium, nickel, silicon, silver and lithium.7. A method for making a net shape article of an aluminum alloycontaining beryllium comprising the steps of:(a) providing a solidaluminum component and a solid beryllium component to form an alloycharge; (b) melting the charge of step (a) in a refractory-lined furnacepot within a vacuum melting furnace; (c) pouring the liquid melt fromstep (b) into a disposable shell mold; (d) freezing said melt withinsaid disposable shell mold; (e) detaching said disposable shell mold toyield a net-shape casting; and (f) removing gates, sprues and excessalloy materials to yield a net shape article.
 8. The method of claim 7,wherein the resulting article has a modulus of elasticity at least 25percent higher than that of aluminum.
 9. The method of claim 7, whereinthe resulting aluminum alloy containing beryllium comprises about 5 toabout 80 weight percent beryllium.
 10. The method of claim 9, whereinsaid article comprises about 5 to about 80 weight percent berylliumdispersed in substantially pure aluminum.
 11. The method of claim 9,wherein said solid aluminum component of step (a) is an aluminum-richcomposition and said net shape article comprises about 5 to about 80weight percent beryllium dispersed in said aluminum-rich composition.12. The method of claim 11, wherein said aluminum-rich compositioncontains an element selected from the group consisting of magnesium,nickel, silicon, silver and lithium.